The present invention relates to optical media and, more particularly, to refractive index gratings incorporated into optical fibers for modulating, filtering, and altering light transmitted through the fibers.
Optical fibers have long been used to transmit light signals with little loss in intensity over great distances. An important problem, however, has been the ever increasing need for faster data transmission. One approach to increasing the transmission capacity of the currently installed base of optical fiber transmission lines is to use wavelength division multiplexing (WDM). Using this approach, a single optical fiber may carry several channels of data simultaneously, with each channel using a slightly different wavelength of light.
With the advent of Bragg gratings that can be incorporated directly into the optical fiber itself, it became possible to build filters to de-multiplex the various channels of data. However, the present generation of wavelength division multiplexing fiber gratings are primarily passive and are selective over a single, fixed, narrow band of wavelengths. Moreover, Bragg gratings are temperature and strain sensitive. Thus, their selectivity and efficiency may be reduced under certain conditions, and there is presently no way, other than adding an additional filter into the transmission line, to correct artifacts created when the grating is operated outside of its design temperature range, or where, because of routing of the fiber, strain is induced in the grating.
Another problem is that even the best optical fibers exhibit some loss of light, primarily due to absorption of the light within the fiber. To solve this problem, amplifiers are typically placed along a transmission line to amplify the light in the fiber to maintain the amplitude of the light at an optimum level. Currently available amplifiers, however, amplify different wavelengths by different amounts, which makes them less than ideal for amplifying WDM lines having multiple channels at different wavelengths. Ideally the gain of each amplifier should be the same for all channels, if the signals all have the same strength. Alternatively, the gain should be adjustable for each channel to accommodate variations in signal strength caused by changes in the operating environment, such as the ambient temperature, or changes causes by fiber strain, or the distance traveled by the signals in the fiber, since signals traveling a relatively longer distance will have decreased amplitude compared to signals traveling a shorter distance.
While in certain cases the sensitivity of fiber gratings to environmental factors is disadvantageous, that same sensitivity may be used to construct robust sensors, since the fiber itself, being manufactured of doped fused silica, is relatively unaffected by radiation and electromagnetic fields. Temperature and stress sensors using optical fibers incorporating fiber gratings are well known. What has been difficult to achieve, however, are sensors that can detect the presence of chemicals or radiation, because the light passing through the core of the optical fiber is isolated from the chemical by the cladding of the optical fiber, and the core of the fiber is typically unaffected by radiation.
Much of the difficulty in addressing the shortcomings of currently available gratings and amplifiers, or manufacturing sensors that detect environmental variables other than temperature or strain, stems from the very characteristic of the optical fibers that make them so useful as a transmission medium. Once a light beam is coupled into the core of an optical fiber, the light beam cannot easily be modified without extracting the light beam from the fiber. The process of light extraction and modification often requires costly electronic equipment and careful optical alignment, both of which are prone to failure. One approach avoiding the extraction of light from the core of the fiber has been to leave the light in the core and introduce a quadratic susceptibility into the core by poling the fiber in the presence of heat or ultraviolet light to make the fiber into an electrically controlled modulator. This process, however, is complicated, time consuming, and the resulting device operates only at frequencies below one megahertz. Moreover, the electrodes necessary to induce the quadratic susceptibility are difficult to fabricate and the resulting device normally requires modulating voltages in excess of 100 volts. Furthermore, the induced non-linearity using this technique is small and relatively unstable.
More recently, an acoustic fiber modulator utilizing sound energy to induce modulation of light within the core of a fiber has been reported. However, this approach is complex, requires a separate transducer, and still does not modulate at frequencies high enough to be commercially useful.
Various techniques have been developed to extract light from the core of a fiber so that the light may be modulated or amplified. In one approach, part of the cladding surrounding the core of the optical fiber is polished away on one side of the fiber so that a portion of the light in the core can be coupled out of the fiber. It is also well known that bending a fiber will cause some of the light transmitted through the core of the fiber to be coupled into the cladding of the fiber, where it becomes more accessible. However, both of these techniques are disadvantageous, either because of the difficulty and cost to implement, or because the technique damages the fiber.
What has been needed, and heretofore unavailable, has been an inexpensive, yet highly reliable system for interacting with the light passing through an optical fiber to provide for active processing of the optical signals transmitted through the fiber. Additionally, there is a need for highly sensitive, yet durable, reliable and inexpensive environmental sensors for detecting and measuring the presence of otherwise difficult to monitor substances, such as gases, chemicals or radiation in situations that would destroy other types of sensors or render them useless after only a short period of time. The present invention fills this need.